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Chapter 3: Educational Lay-ups - Started 7/19/06 and finished 8/2/06.

8/5/06 - Chapter 3 consists of three practice layups.  Each practice layup is designed to teach some of the various techniques that are used in building a composite aircraft.

But first some background on the types of material used to create the layups.  A majority of the aircraft parts are made from a composite sandwich structure that consists of a foam core with fiberglass cloth placed on either side of the core.  The fiberglass is held in place on the core by cured epoxy. 

Two types of fiberglass are used to build the airplane.  Unidirectional (Und) fiberglass is glass cloth that has all of the load carrying fibers running in only one direction along the length of the cloth.  Bi-directional (Bid) fiberglass is glass cloth that has half of the load carrying fibers running longitudinally, with the other half running perpendicular to the longitudinal fibers.

The pictures below show how the glass and foam look before the glass is saturated with epoxy.


 

a piece of unidirectional fiberglass


a piece of Bi-directional fiberglass


A piece of foam similar in shape to the piece used in the confidence layup below.

The foam is easy to shape, can be cut with a knife and can be sanded into smooth shapes by using other pieces of scrap foam.

1.  Flat Layup

This is a picture of the first practice layup.  It is made up of 6 plies of bid fiberglass and was trimmed after cure to a 10x16 inch rectangle using a Fein Multimaster tool.  I am using the MGS 285 epoxy system with only the slow hardener at this point.

The purpose of this layup is to develop a feel for the amount of epoxy that is needed to wet out the glass properly.  The plans say that the finished piece should weigh somewhere between 10.5 and 12.5 ounces in order to be considered acceptable. This part weighed in at 11.5 ounces, so I was satisfied with my first layup attempt. 

I did this layup on a layer of 4 mil plastic that I taped to my work table.  The biggest lesson I learned from this layup is that any bumps or creases in the plastic will show up on the surface of the layup. 

Before I started I thought that the weight of the glass and epoxy would smooth out the crease in the plastic.  However, this was not the case.  No amount of squeeging would make the crease stay flat.  The picture below shows how the glass conformed to the crease.

2.  Confidence Layup

This layup is designed to show the strength of the composite sandwich structure and is typical of the sandwich structure that makes up a majority of the plane's components.

This part consists of a urethane foam core that is .5 inch thick, 1.5 inches wide and 14 inches long and 4 plies of Bid and 4 plies of Und.  Two plies each of Und and Bid are placed below the foam core with the remaining plies placed above the core.

After cure the glass is trimmed to be 2.5 inches wide and 16 inches long.  Again, I waited until full cure occurred and then trimmed the glass using the multimaster tool.  It cuts through the glass very easily and leaves a relatively smooth edge with no jagged glass needles to stick you in your fingers. 

After full cure you place the center of the layup on a broomstick running perpendicular to the piece and then, while pushing down with your hands on both ends of the layup, place all of your weight on the piece to see the strength of the composite sandwich structure.  This part was able to take all my weight without hardly bending.
 
I did have a problem getting the glass to lay down at the tapered ends of the foam.  When I finished the layup the glass was conforming to the surface of the foam.  However, when I checked the layup the next morning I discovered that a void had formed between the glass layers at each end of the core. 

 I will try laying a piece of saran wrap over the last ply of glass the next time I am faced with a similar situation.
 
3. Book End Layup
 
This layup is designed to teach how to contour the foam, make glass to glass bonds, do layups over inside and outside corners and apply micro beads and flox corners.  
 
Micro is a filler material consisting of tiny glass bubbles and epoxy and is used to fill voids in the foam surface before the fiberglass is layed over the foam.  Micro looks very much like powdered sugar before it is added to the epoxy.  After micro powder is added to epoxy it is called a micro slurry and it can be made to varying consistencies depending on the particular application.
 
Flox is similar to micro, only finely flocked cotton replaces the glass bubbles.  Flox is used when a stronger joint is needed, typically between two adjoining pieces of fiberglass. 
 
This project must be done in 3 layups, allowing the epoxy to fully cure between each layup.  Because I used the slow hardener it took me several days to complete the book end pictured below. 

This is a picture of me, my daughter and our bulldog.
 

I used a utility knife to trim the third layup after partial cure to determine if I would like this method better than using the Fein after full cure.  It worked well, but not as well as the Fein, as the knife trim did leave some small glass needles that had to be sanded down after the epoxy fully cured.
 
After I build the the front seat bulkhead I plan on showing these pieces and the bulkhead to some local builders/flyers so that they can critique my work to this point.


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